The advantages of using composite honeycomb core in low-density sandwich structures have been well documented and understood for many years. Unfortunately the technology designed to produce such cores has not kept up with the advances in composite material technologies. The standard methods of composite core manufacturing tend to exhibit such problems as release residue in the inner core structure, non-uniform node adhesion, inaccurate geometrical structure, as well as a non-constant tg (glass transition temperature) throughout the volume. Due to these inadequacies of construction the structural and dielectrical properties of the core tend to be compromised.
The standard method for creating a composite core involves the stacking of aluminum rods over sheets of unidirectional or fabric prepreg. This process creates a large block of mostly aluminum that is generally heated in some form of a standard oven using convection as the method of heat transfer to the outer edges of the rods and then through conduction for the center of the block. The temperature curve with respect to block thickness will take tens of minutes to level off resulting in a non-even cure rate. Thus when the resin in the center of the block is just starting to advance the external edges could already be fully cured. This process will never be able to yield a core that has an even tg. The energy required to heat large blocks, tens of cubic feet, becomes astronomically large. When heating a large block the thermal difference from exterior to interior will result in differentials of thermal expansion. These differentials will show themselves as points of node separation as well as non-uniform cell size in which the structural, dielectrical and thermal advantages of the core are compromised.
A very large problem common with the aluminum rod composite core manufacturing process is due to the release agent applied to each aluminum rod which inherently a necessary step required to extract the rods from the core. The release agent tends to leave a silicon coating on the core. Structurally this inhibits a full bond between the core and skin of the structure. The residue also adversely affects the dielectrical properties of the core.
The lack on uniformity in cell geometry is a key factor in inhibiting the progression of composite honeycomb being applied to the field of R.F. cancellation in the aerospace market. Presently syntactic core is generally used due to its ease in dielectrical loading but structurally honeycomb core material is much more advantages. To control the dielectrical properties of composite honeycomb core, the core is created through an older method than previously stated, instead of using aluminum rods a nomex fabric is lightly impregnated with a resin, bonded at nodes and expanded much like the creation of an aluminum honeycomb core. The nomex style core is geometrically non-accurate and dialectically useless for R.F. cancellation. To make the core useful it is repeatedly dipped in a resin of particular dialectic properties until the desired effects are achieved. This works to a point but again dose not take advantage of the uniformity and controllability of the advance composite prepregs.
To overcome the aforementioned shortcomings, the present invention comprises a honeycomb production apparatus that will eliminate the present problems of composite core manufacturing and take full advantage of the advances in modern composite technologies.
The apparatus of the present invention produces composite honeycomb core through the corrugation of individual sheets of resin-impregnated fibers or fabric (prepreg). The corrugated sheets are stacked and adhered together using a node adhesive film. The resulting core will be extremely uniform, heat formable, absent of release residue, capable of extremely large ribbon lengths and will have dielectric properties controlled by the resin and fiber content.